1. Field of the Invention
The present invention concerns a novel ionically conductive macromolecular material comprising a novel family of salts in solution in a polymer.
Ionically conductive macromolecular materials such as described in European patent 013 199 are constituted by a solid solution of an alkaline metal salt in a polymer the chain of which recurrently comprises at least one hetero-atom having the solvating properties of lithium salts. A family of preferred polymers of this European patent includes the polyethers, in particular, polyoxyethylene and/or ethylene oxide copolymers.
The salt to be dissolved must present certain specific qualities. It must be electrochemically stable and have a high ionic disassociation rate. Furthermore, it must be capable of forming with the polymer a solid solution that remains homogeneous in wide concentration and temperature ranges in order to prevent any precipitation of the non conductive crystalline phases within the electrolyte.
Due to the model of the electrochemical reactions occurring in the electrochemical generators using electrolytes obtained according to European patent 013 199, it is necessary that the salt confers upon the electrolyte a very high cationic conductivity with respect to the anionic conductivity, the ideal situation being that the conductivity is exclusively cationic.
It has thus been proposed to reduce the mobility of the anions by using salts having a high molecular weight or salts that are very bulky such as those described in French patent applications 82 09538, 82 09539 and 82 04540 filed on June 1, 1982 in the name of Agence Nationale de Valorisation de la Recherche or in French patent application 82 05623 filed by the Agence Nationale de Valorisation de la Recherche on Mar. 18, 1982.
The use of these large anion salts is limited by the fact that if it is desired to reach a reasonable concentration of lithium cations, it is necessary to introduce into the polymer a strong concentration of the anion associated to the lithium, whereby the dielectric constant of the medium and thus its solvation and dissociation power may be impaired, this being, in particular the case of anions having a very high molecular weight.
The object of the invention is thus to provide a novel family of salts the anions of which are the least mobile when in solution in a polymer and which are completely compatible with the polymer.
With this object in view, the invention provides an ionically conductive macromolecular material constituted by at least one salt in solution in a macromolecular weight, the said macromolecular material being mainly constituted by an amorphous structure of the polyether type. According to the invention, the said salt is a salt represented by formula (I): EQU R--F--M (I)
in which:
M is an alkali metal, in particular lithium; PA1 R is a polyether type structure; PA1 F represents a group conferring on the polyether type structure one or several functions selected from among alcoholates, sulfonates, sulfates, phosphates, phosphonates, amides, carboxylates, borates, aluminates, thiophosphates, perhalogenosulphonates, in particular the dihalogenosulphonates, and the perhalogenocarboxylates. PA1 M has the same significance as in formula (I); and PA1 X represents an anion of a strong acid. PA1 A is a high weight molecular polyether chain; PA1 R, F and X have the same significance as in the formula (I), the molecular weight of R being lower than of A; PA1 x is the number of graftings carried by the polyether chain.
The polyethers of the anions of the salts according to the invention can have high molecular weights. They do not impair the solvating power of the macromolecular material in which the salts are in solution, since they have the same structure as the said materal.
Due to this high molecular weight, the R-F-anion is almost immobile in the macromolecular material, which means that the solid solution obtained presents an ionic conduction of an essentially cationic nature.
With the alcoholate functions, it is possible to obtain according to the present invention salt functions of alkali metals of acid complexes according to formula (II): EQU --OM.sub.t X.sup.- n,M.sup.+ (II)
These acids being obtained by reacting an alkali alcoholate --OM with a Lewis acid of the Friedel and Krafts M.sub.t X.sub.n type where M.sub.t represents in the Periodic Classification of Elements, an element of column 3, such as for example boron, or an element of column 4 such as for example tin, or an element of column 5 such as arsenic.
Among the preferred Lewis acids can be selected BF.sub.3 , BCl.sub.3, SbCl.sub.5, SbCl.sub.3, AsF.sub.5, TiCl.sub.4, SnCl.sub.4. A salt according to formula (III) is thus obtained: EQU R--OM.sub.t X.sup.-.sub.n,M.sup.+ (III)
According to one preferred embodiment of the invention, the function conferred by the group F, or the ionisable function, is a carboxylated function represented by the formula: EQU --CF.sub.2 CO.sub.2 M (V)
and is derived from a carboxylic acid fluorated in .alpha. of formula (VI): EQU --CF.sub.2 CO.sub.2 H
M having the same significance as in formula (I),
so that a salt of formula (X) is obtained: EQU R--CF.sub.2 CO.sub.2 M
in which R has the same significance as in formula (I).
According to another embodiment of the invention, the polyether chain of the anion is itself constituted by a functionalized polyether of formula (XI): EQU R.sub.1 --F.sub.1 --M.sub.1 (XI)
in which R.sub.1, F.sub.1 and M.sub.1 have the same significances as in formula (I), so that the anion, object of this embodiment according to the invention, can be represented by the formula (XII): EQU M.sub.1 --F.sub.1 --M.sub.1 --F--M (XII)
Preferably, M.sub.1, F.sub.1 and F, M are identical.
These salts are dianionic and each anionic group is separated from the other by a polyether chain. They present a particularly low mobility due to the increase of the molecular tangling to which the materials are subject.
According to another characteristic of the invention, the macromolecular material comprises, furthermore, a second salt represented by the formula (XIII): EQU MX (XIII)
in which:
The preferred anions are those described in European patent application 013 199 and in particular anion C10.sub.4.sup.-.
Another advantage of the salts according to the invention and the ionically conductive macromolecular materials obtained thereby lies in the improvement of the wetting qualities presented by the electrolyte with respect to the electrodes materials in the composite electrodes, i.e. in the electrode materials in which each grain of active material is completely embedded in a ionically conductive macromolecular material, whether this material is the same as that constituting the electrolyte or whether it is different therefrom by the nature and the constitution of the chain.
But according to a further characteristic of the invention, the salt represented by formula (I) is grafted onto at least one part of the macromolecular material constituting the electrolyte, and each of the macromolecules thus grafted may carry one or several chains at the anionic end.
In order to obtain this grafting, it is possible for example to carry out a polycondensation of a polyetherglycol of a polyether chain, one end of which carries the anion and the other end of which carries an alcohol group or any other group able to react with a polycondensation agent.
Such types of polycondensation are, for example, described in French patent application no. 84 13925 dated Sept. 11, 1984.
According to a first variant of this embodiment, the first and the second structure of the polyether type derived from one monomer structural unit or identical monomer units and the first structure is linear and carries a single second structure grafted on the first.
According to a second variant of the same embodiment, at least two second structures are grafted onto the first structure, the copolymer thus obtained having a combshaped structure.
Such a macromolecular material obtained according to the present invention, can be represented by the formula: EQU A(R--F--X)x (XV)
formula in which:
The essential advantage of such a process lies in the limitation of the mobility of the anionic site with respect to the chain of the solvent polymer. In fact, the anionic site is bound to the main chain by the intermediary of the polyether chain of the anion (chain-R- formula I) and its mobility is physically limited by the mobility of the polyether chain and of the anion and its stretching properties. Furthermore, with such a process, it is possible to increase the salt concentration in the polymer solvent without risk of increasing the vitreous transition temperature, as would occur if the anionic grouping were fixed directly onto the chain of the polymer solvent without interposing the polyether group.